Printed circuit boards and cards, also known as printed wiring boards, are ubiquitous in the 21st century. There are many designs of printed circuit boards, such as those described in M. W. Jawitz “Printed Circuit Board Materials Handbook”, McGraw-Hill (1997), Clyde Coombs, Jr., “Printed Circuits Handbook”, McGraw-Hill (1996), and IPC/JPCA-2315 Standard “Design Guide for High Density Interconnects and Microvias”, and many fabrication procedures. In general, they comprise an electrically insulating dielectric layer upon which is disposed a layer of a metal. The metal can be laminated, glued, sputtered, or plated on to the printed circuit boards substrate. In-typical commercial practice, the dielectric layer is made up of a fiber-reinforced composite sheet whereof the matrix resin is a cross-linked organic polymer such as epoxy. The metal-coated dielectric layer thus prepared is then subjected to a series of steps, all well-known in the art, to leave a circuit pattern upon the electrically insulating dielectric layer forming a so-called “circuitized layer”. The circuit pattern serves to connect the various electronic components which will then be added to make up the desired electronic device. Such a circuitized layer can be used alone or in a multilayer stack as described in IPC/JPCA-2315 with vias and interlayer connections. Other layers in such multilayer stacks may optionally use other dielectric materials. It is known in the art to employ an uncured (i.e., uncrosslinked) dielectric “prepreg” as an interlayer adhesive.
The art further teaches the use of dielectric layers employed to form an integrated package with an integrated circuit chip (IC). For IC's with high input/ouput counts, these IC packages may consist of one or more bare IC chips attached to a circuitized substrate structure similar in construction to printed circuit boards but smaller in size. These circuitized substrate structures so employed are typically called IC chip substrates, chip carriers, or chip package interposers.
The requirements on the dielectric materials employed in printed circuit boards are stringent, involving both electrical and thermo-mechanical requirements. For this reason dielectric layers employed in the art are nearly always composite sheets made up of a reinforcing fabric impregnated with an organic polymer resin Generally the resin so-employed is cross-linked.
Suitable materials must be strong, easy to handle, and have a minimum of defects on the scale of several microns or less. The distribution of matrix resin or polymer and reinforcing fiber (in the form of a woven or non-woven fabric, sheet or paper) must be sufficiently uniform to afford minimum distortion and very high dimensional stability through a series of manufacturing steps which include solder bath temperatures typically in the range of 200° C.-300° C. They must offer excellent adhesion to metal. They must be chemically inert to the many solvents to which they will be exposed during the manufacturing operation.
Furthermore, when an electronic signal propagates through a conductor, an electromagnetic field permeates into the dielectric material adjacent to the conductor. The interaction between the dielectric material and this electromagnetic field affects the propagation properties of the signal. These interactions are especially important for high frequency applications. For these reasons, the dielectric properties of the composite sheet are important. In particular the dielectric constant determines the speed of signal propagation through the circuit and affects signal cross-talk between circuits, and the dissipation factor determines signal loss.
Current commercial printed circuit boards composite sheets include fiberglass or para-aramid fiber reinforcements. Non-woven para-aramid fabric such as Thermount® laminates available from DuPont, are typically embedded in a matrix of an epoxy resin. However, it is found that the epoxy composites do not exhibit desirable dielectric constants and dissipation factors for use at frequencies of ca. 1 GHz and higher. There is considerable incentive in the art to provide printed circuit boards which retain the desirable properties of the epoxy composites such as dimensional stability and good adhesion to metal while providing a decrease in dielectric constant and dissipation factor.
Dragone et al, U.S. Pat. No. 6,119,575, discloses a composite of para-aramid fibers in various configurations in a matrix of styrene butadiene styrene block copolymers said composite being employed as a component in body armor. No cross-linking is provided for in Dragone.
Landi et al, U.S. Pat. No. 5,223,568, discloses a process for producing a hard shaped molded article made by combining polybutadiene or polyisoprene with a solid butadiene or isoprene polymer, followed by shaping and heat curing. Para-aramid fibers are employed only as a minor, filler component.
Landi, U.S. Pat. No. 6,048,807, and St. Lawrence et al, U.S. Pat. No. 6,071,836, disclose polybutadiene and polyisoprene thermosetting compositions for use as an electrical substrate material. Disclosed are compositions comprising polybutadiene or polyisoprene, para-aramid fabric reinforcement, and an unsaturated block copolymer having blocks of polybutadiene and blocks of polystryene. In one embodiment which does not include an para-aramid reinforcement, the concentration of the block copolymer exceeds that of the homopolymer ingredient. It is clear in Landi and in St. Lawrence that the block copolymer is a non-essential additive since preferred embodiments are described which omit the block copolymer. Furthermore the inclusion of high levels of particulate fillers is stated to be essential.
Porter, U.S. Pat. No. 3,149,182, discloses a process for forming block copolymers using organolithium initiators.
Holden et al, U.S. Pat. No. 3,265,765 discloses block copolymers of monovinyl aromatic hydrocarbons and conjugated dienes.
Tung et al, U.S. Pat. No. 4,431,777, discloses block copolymers of diene having terminal end blocks of a random copoymer of styrene or alkylstyrene and an alphamethylstyrene.
Para-aramid fibers are disclosed in U.S. Pat. Nos. 4,172,938; 3,869,429; 3,819,587; 3,673,143; 3,354,127; and 3,094,511. WO 99/05360 discloses para-aramid papers formed from a mixture of para-aramid short fibers also known as floc, and fibrids. WO 94/23553 discloses a nonwoven para-aramid sheet useful as reinforcement in printed circuit board laminates.